Not married yet, but getting married this weekend! But my fiance and I wish we put more honeymoon-specific things on our registry. Someone just paid for us to go on a sunset cruise next week and it was such a great idea that we wished we had thought of doing more things like that.
See if you can buy tickets to an event or vouchers for something at your honeymoon in advance!
The biggest mistake would be registering at a traditional store like Bed, Bath, and Beyond.. Do yourselves a favor and register on Amazon. This way, you're only limited by your imagination, and you get far more selection this way. It's also way easier for people to have the items shipped to their house instead of potentially going to a store.
It was one of the smartest moves we made before our wedding, and we scored a bunch of camping gear, my wife got some fancy shoes, I got a nice fishing pole, and we still got the traditional kitchen junk.